An atmospheric pressure ionization mass spectrometer analyzes the mass of ions by introducing ions generated at atmospheric pressure into a vacuum system. Among atmospheric pressure ionization methods that are widely used are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI).
In ESI, a sample solution is flowed through a sample spray nozzle (i.e., capillary) to which a high voltage is applied, so as to be sprayed and form charged droplets, and then, the charged droplets repeatedly undergo evaporation and fission to generate ions. In ESI, a method is also used that includes coaxially arranging a nebulizer gas nozzle around the outer circumference of the sample spray nozzle so that finer charged droplets are sprayed with blowing of nebulizer gas. When the liquid flow rate is high, in particular, a method of spraying a large amount of heated gas (i.e., heating gas) to promote evaporation and vaporization of the droplets is also used in combination. ESI is an ionization method that can be applied to a high-molecular-weight sample with a high molecular weight, a highly polar sample with high polarity, and the like.
APCI is a method of ionizing sample molecules, which have been obtained by heating and vaporizing a sample solution, using corona discharge. In this method, electric charges move between the sample molecules and the primary ions generated by the corona discharge so that the sample molecules are ionized. APCI can be applied to even a low-molecular-weight sample with a lower molecular weight than that in ESI or a low polarity sample with lower polarity than that in ESI.
Therefore, it is necessary to selectively use the ionization methods depending on samples to be analyzed. For such reasons, if a plurality of ionization methods (i.e., ESI and APCI) that are based on different ionization principles can be implemented using a single ion source, it becomes possible to expand the range of substances to be measured.
Patent Literature 1 describes a method of switching between two ionization methods, specifically, a method of switching an ionization method from ESI to APCI or vice versa by manually switching a probe from an ESI probe to an APCI probe or vice versa.
Patent Literature 2 and Patent Literature 3 each propose a method of executing ESI and APCI using an ion source with the same configuration without switching a probe or the like. An electrostatic spray portion of ESI and a needle electrode of APCI are arranged in the same space, and ESI ionization and APCI ionization are executed concurrently.
Patent Literature 4 describes a configuration in which an atomization chamber that is movable in the axial direction of an ionization probe (i.e., needle) is provided, and an ionization method is switched by moving the atomization chamber between ESI and APCI. The needle and the atomization chamber are moved by a movement mechanism so that an end of the needle is arranged such that it protrudes forward beyond the atomization chamber in ESI and is arranged within the atomization chamber in APCI. With this method, the ionization method can be easily switched in a short time.